ll known than him. So Brahe assigned him the job of understanding the orbit of Mars because it gave Brahe much difficulty, and would keep Kepler occupied while Brahe worked on his theory of the solar system. Brahe also kept other data from Kepler hoping that he would be the one to discover the orbits of the planets before Kepler did. In 1601, when Brahe died Kepler received all of Brahe’s data. Whether he obtained this data legally or not is still in debate today, however, it is fortunate that he obtained this data. (Silverberg, 160)When Kepler first began his work on the orbit of Mars he was under the assumption, as many scientists were, that the planetary orbits were circular, and that the Sun was at the center of the orbits. This type of system is called a heliocentric system. Also at this time only six planets were known. When Kepler obtained Brahe’s data he discovered that the orbits were not perfect circles, but instead were ellipses that were only slightly flattened. The reason nobody else realized this was because the orbits were so slightly elliptical that extensive investigation and data would be needed to show this. It also turned out that the reason the orbit of Mars was very difficult to understand was because its orbit was more eccentric than the other planets that Kepler and Brahe had data about. To understand a lot of Kepler’s work you must first understand some simple ideas about an ellipse. An ellipse is defined, as the locus of all points, whose sum of distances from two fixed points, also called the foci, is a constant. Below is an example: (http://csep10.phys.utk.edu/astr161/lect/history/kepler.html) In the example the Xs represent the foci. No matter where the point is on the path of the ellipse, the sum of the distances from the foci will be the same. The distance between the foci is what gives the ellipse its shape, or eccentricity. As the foci move closer together the ellipse becomes more ...
